(a) Field of the Invention
A positive active material for a rechargeable lithium battery, a method for manufacturing the same, and a rechargeable lithium battery including the same are disclosed.
(b) Description of the Related Art
In recent times, portable electronic equipment with a reduced size and weight has been increasingly used in accordance with developments in the electronics industry.
Batteries generate electrical power using electrochemical reaction materials for a positive electrode and a negative electrode. Lithium rechargeable batteries generate electrical energy from changes of chemical potential during intercalation/deintercalation of lithium ions at the positive and negative electrodes.
Lithium rechargeable batteries use materials that reversibly intercalate or deintercalate lithium ions during charge and discharge reactions for both positive and negative active materials, and contain an electrolyte between the positive electrode and the negative electrode.
For positive active materials of a rechargeable lithium battery, lithium-transition element composite oxides being capable of intercalating lithium such as LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoxO2 (0<x<1), LiMnO2, and the like have been researched.
Among the above materials, a lithium nickel-based oxide is less expensive than a cobalt-based oxide but secures high discharge capacity when charged at 4.3 V, and thus, a doped lithium nickel-based oxide realizes reversible capacity near about 200 mAh/g, which is greater than capacity of the LiCoO2 (about 165 mAh/g). Accordingly, a lithium nickel-based positive active material has improved energy density despite a somewhat low discharge voltage and volumetric density, and thus is commercially available for a high-capacity battery.
In particular, active research on a nickel-rich-based positive active material has been recently made to develop a high-capacity battery.
However, the nickel-rich-based positive active material has the largest problem of structure stability at a high temperature and lithium impurities such as Li2CO3 and LiOH remaining on the surface during synthesis. The lithium impurities remaining on the surface react with CO2 or H2O in the air and form Li2CO3. In addition, Ni3+ ions are reduced into Ni2+ ions during exposure in the air for a long time, under an increasing partial pressure of CO2, or during an electrochemical reaction, which directly decreases capacity.
In addition, lithium impurities act as a factor of determining pH of an active material, and an active material having high pH causes gelation during manufacture of an electrode slurry and deteriorates uniformity of an electrode plate and thus is not appropriate for commercialization. Furthermore, the Li2CO3 has a decomposition reaction during an electrochemical reaction and mainly generates gas as well as causes a problem of forming initial irreversible capacity, hindering movement of lithium ions on the surface, and the like.
Accordingly, a great deal of research on a surface treatment to secure structure stability of the nickel-based positive active material and to suppress a side reaction on the surface has been made. A representative surface treatment material for securing the surface stability includes various metals such as Ag and the like, metal oxides such as Al2O3, ZrO2, CeO2, and the like, metal phosphates, metal fluorides such as ZrF2, AlF3, SrF2, and the like, carbon compounds, and the like. However, a conventional surface treatment material acts as an insulator and is unfavorable in terms of battery conductivity and lithium ion conductivity, and thus causes a problem of deteriorating initial capacity, increasing initial resistance, or the like. In addition, the lithium impurities remaining on the surface are not removed through coating, and thus attempts to remove them through reheat-treatment, washing, or the like have been made, but they are recrystallized during cooling when reheat-treated and cause another problem of controlling moisture when massively washed.